Urban legends
Two unfounded urban legends about Mountain Dew exist. One is that it causes shrunken testicles and/or penis size. The other is that it lowers sperm count. Both myths are typically attributed to the dye Yellow #5 (tartrazine).
Yellow #5 has never been scientifically linked to any of the alleged effects in the legends, nor has any other component of the drink. Thus, there is no evidence that Mountain Dew is any more hazardous (or beneficial) to health than other caffeinated, sugar-sweetened soft drinks, provided one is not allergic to Yellow #5.